Embodiments of the present invention relate to an apparatus including inductive coupling communication circuitry configured to communicate in peer-to-peer mode with an identical apparatus, the communication circuitry including a communication controller and a planar antenna coil tuned to a working frequency, linked to the controller by way of two connection points and including N loops arranged in series between the two connection points.
Embodiments of the present invention also relate to a method of limiting a frequency detuning between first and second identical planar antenna coils turned to a working frequency when the antenna coils are arranged opposite each other at a distance less than 10 millimeters.
Embodiments of the present invention also relate to a method for establishing a communication by inductive coupling between two identical apparatuses, each including inductive coupling communication circuitry configured to communicate in peer-to-peer mode.
Embodiments of the present invention relate in a general manner to inductive coupling communication techniques also known as “NFC” (Near Field Communications).
A communication by inductive coupling generally uses a passive NFC device and an active NFC device, each equipped with an antenna coil tuned to a working frequency, for example 13.56 MHz. The active device emits a magnetic field oscillating at the working frequency, and sends data to the passive device by modulating the magnetic field. The passive device receives a voltage induced by the magnetic field, which it demodulates to extract the data contained therein, and sends data to the active device by charge modulation.
Apparatuses equipped with NFC devices, in particular NFC mobile telephones, generally have three operation modes: a reader mode, a card emulation mode, and a peer-to-peer mode. Embodiments of the present invention relate particularly to communications in peer-to-peer mode between two identical apparatuses.
During a communication in peer-to-peer mode, two apparatuses are arranged opposite each other, one being in active mode and emitting the magnetic field, the other being in passive mode and receiving the magnetic field in the form of an induced voltage in its antenna coil.
The conception of the antenna coil should generally take into consideration standards and constraints applicable to contactless cards, of those applicable to contactless card readers, as well as constraints specific to the peer-to-peer operating mode.
The standard ISO/IEC 7810 specifies a rectangular format “ID-1” for contactless cards used mainly in the transportation and payment sectors.
For the reading of contactless cards, the format of the reader and of its antenna is not standardized. However, the magnetic field produced by the reader's antenna coil must respect certain limits and its performances are verified during the certification of the reader. For applications in the transportation sector, the certification tests are based on the standard ISO/IEC 10373-6, whereas the standard “EMV contactless protocol 2.0” is generally applied in the payment sector.
The antenna coil of an NFC apparatus is generally planar and extends proximate to the internal face of a wall of the housing of the apparatus, for example its back wall. A communication in peer-to-peer mode thus requires placing the walls, proximate to the antenna coils, of the two devices mutually opposite each other.
In such a configuration, the antenna coils of the two apparatuses may be closely juxtaposed, their respective loops being mutually opposite. As the back walls of the housings are generally thin, they may be very close to one another, generally at a distance less than 10 millimeters.
Embodiments of the present invention include the observation that when a small distance separates two identical antenna coils placed opposite each other, a strong inductive coupling results, which detunes the frequency of each antenna coil and causes its Q factor to drop. This has the consequence of greatly reducing the amplitude of both the signal transmitted by the emitting antenna coil and the voltage received by the receiving antenna coil. Unexpectedly, these losses may be such that they prevent a communication between the two apparatuses, despite the fact that they are very close to one another.
As an example, the transmission losses were evaluated between two identical antenna coils opposite each other, each antenna including two imbricated planar loops extending along a rectangular outline of 31 millimeters (mm) in length and of 51 mm in width, the planes of the antenna coils being separated by a distance of 5 mm then by 1 mm. With a separation distance of 5 mm, the maximum transmission is obtained at a frequency of around 14 MHz instead of at 13.56 MHz, which reveals a slight detuning caused by the coupling, and the transmission losses are on the order of −6 dB, signifying that only 50% of the magnetic field is transmitted.
With a separation distance of 1 mm, the antenna coils are completely detuned with a low Q factor and the maximum transmission occurring at around 23 MHz. Thus, bringing the two antenna coils closer together leads in this case to a large shift of their tuning frequency, here from 13.56 MHz to 23 Mhz. Consequently, since the antenna coils are no longer tuned to the working frequency of 13.56 MHz, the transmission losses at 13.56 MHz greatly increase, and are on the order of −10 dB: only 32% of the magnetic field is transmitted.
It may therefore be desired to improve the quality of a communication by inductive coupling between two identical planar antenna coils turned to a working frequency, in particular when the antenna coils are arranged opposite each other at a distance less than 10 millimeters.
The patent application US 2004/0256468 discloses antenna coils integrated within an NFC telephone destined to read contactless cards and deprived of the peer-to-peer communication mode. The antenna coils shown in the FIGS. 6, 7A, and 8 of this document present a symmetric structure relative to a longitudinal Z-axis of the telephone (FIG. 11), and an asymmetrical structure relative to an X-axis perpendicular to the Z-axis. The antenna coils shown in the FIGS. 9 and 10 of this document include four loops and present an asymmetrical form relative to the longitudinal Z-axis, such that a contactless card can be read even when it is arranged at a point distant from the center of the antenna coil, for example above a keypad 71 of the telephone (FIG. 11).